Semiconductor devices such as a transistor, an IC and an LSI are conventionally produced by encapsulating a semiconductor element with a ceramic, but in light of cost and productivity, they are popularly produced by encapsulating with resin. In this resin encapsulation, an epoxy resin has conventionally been used and good results have been obtained.
With the increase in a degree of integration through technical innovation in the semiconductor field, the size of the semiconductor is enlarged, but there is strongly demanded to miniaturize a semiconductor device and to be made it into a thin device. For this reason, the volume capacity which occupies a resin material for encapsulation decreases, and it is required for the encapsulating material to have higher reliability.
Particularly, in recent years, with the large-sizing of the semiconductor element, further improvement on performance for a thermal cycle test (TCT) which is an acceleration test for evaluating a resin for encapsulating a semiconductor material is required. As a process for mounting a semiconductor device on a printed board, a surface mounting has been popularized. For this reason, the encapsulating resin is required to possess a characteristic that when a semiconductor device is soldered in a moisture-absorbed state, no crack or swelling takes place in the semiconductor device.
For these requirements, in order to enhance resistance to thermal stress evaluated in the TCT test, it has conventionally been considered that fine rubber particles are added to an encapsulating resin or an epoxy resin is modified with a silicone compound, to decrease thermal stress.
For the purpose of enhancing the crack resistance when soldering, it has also been considered that an encapsulating resin having a small moisture-absorption is selected and that adhesion to a leadframe is improved, but their effects are yet insufficient.
The present inventors have proposed in U.S. patent application Ser. No. 08/421,253 filed on Apr. 13, 1995 that the surface of plastic package is covered with a metal foil in order to obtain a highly reliable semiconductor device excellent in moisture resistance, resistance to thermal stress, and crack resistance when soldering even when a semiconductor device is made into a thin type. As the process for producing it, the present inventors have proposed that when a resin for encapsulating a semiconductor element is molded, a metal foil material is temporarily fixed on the molding face of a mold, and a resin is then injected, followed by molding to directly fix the metal foil material on the surface of the plastic package. In this case, as a process for the temporary fixation on the molding face of the mold, a process in which an adhesive layer is provided on the face of a metal foil material to be temporarily fixed is proposed. JP-A-63-250846 (the term "JP-A" used herein means a Japanese unexamined patent publication) proposes a process for temporary fixation by physical means, such as vacuum adsorption, magnetic power, and gravity.
The process for producing a semiconductor device comprising temporarily fixing a metal foil material having an adhesive layer for temporary fixation on a mold via the adhesive layer, and injecting an encapsulating resin followed by molding the encapsulating resin has the following disadvantages:
(1) Since the adhesion between the metal foil material and the semiconductor device is insufficient, the metal foil material sometimes peels during molding the encapsulating resin, curing the encapsulating resin after molding, and applying thermal stress such as mounting the semiconductor device on a printed board, and as a result, the effects expected to be obtained by covering the surface of the semiconductor device with the metal foil, e.g., improvement in reliability after the TCT and improvement in crack resistance when soldering and the TCT, are partly impaired. PA1 (2) Unless the tensile characteristics of the metal foil material used are appropriately selected, no effect of reliability nor crack resistance in the TCT can be exhibited. PA1 (3) As for the metal foil material used to temporarily fix on the surface of mold cavity, depending on the type of the adhesive layer formed on the metal foil material, position deviation or mold stain sometimes occurs.